Preparation of 2′-fluoro-2′-alkyl-substituted or other optionally substituted ribofuranosyl pyrimidines and purines and their derivatives

ABSTRACT

The present invention provides (i) processes for preparing a 2′-deoxy-2′-fluoro-2′-methyl-D-ribonolactone derivatives, (ii) conversion of intermediate lactones to nucleosides with potent anti-HCV activity, and their analogues, and (iii) methods to prepare the anti-HCV nucleosides containing the 2′-deoxy-2′-fluoro-2′-C-methyl-ß-D-ribofuranosyl nucleosides from a preformed, preferably naturally-occurring, nucleoside.

CLAIM TO PRIORITY

This application is a continuation of U.S. patent application Ser. No. 14/808,147, filed Jul. 24, 2015, now abandoned, which is a divisional of Ser. No. 13/917,173, filed Jun. 13, 2013, now abandoned, which is a continuation of U.S. patent application Ser. No. 11/225,425, filed Sep. 13, 2005, now U.S. Pat. No. 8,492,539, which claims the benefit of U.S. Provisional Patent Application No. 60/609,783, filed Sep. 14, 2004, U.S. Provisional Patent Application No. 60/610,035, filed Sep. 15, 2004, and U.S. Provisional Patent Application No. 60/666,230, filed Mar. 29, 2005. The entire contents of all of the above-mentioned applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention provides (i) processes for preparing a 2-deoxy-2-fluoro-2-methyl-D-ribonolactone derivatives, (ii) conversion of intermediate lactones to nucleosides with potent anti-HCV activity, and their analogues, and (iii) methods to prepare the anti-HCV nucleosides containing the 2′-deoxy-2′-fluoro-2′-C-methyl-β-D-ribofuranosyl nucleosides from a preformed, preferably naturally-occurring, nucleoside.

BACKGROUND OF THE INVENTION

HCV infection has reached epidemic levels worldwide, and has tragic effects on the infected patients. Presently there is no effective treatment for this infection and the only drugs available for treatment of chronic hepatitis C are various forms of alpha interferon (IFN-α), either alone or in combination with ribavirin. However, the therapeutic value of these treatments has been compromised largely due to adverse effects, which highlights the need for development of additional options for treatment.

HCV is a small, enveloped virus in the Flaviviridae family, with a positive single-stranded RNA genome of ˜9.6 kb within the nucleocapsid. The genome contains a single open reading frame (ORF) encoding a polyprotein of just over 3,000 amino acids, which is cleaved to generate the mature structural and nonstructural viral proteins. ORF is flanked by 5′ and 3′ non-translated regions (NTRs) of a few hundred nucleotides in length, which are important for RNA translation and replication. The translated polyprotein contains the structural core (C) and envelope proteins (E1, E2, p7) at the N-terminus, followed by the nonstructural proteins (NS2, NS3, NS4A, NS4B, NS5A, NS5B). The mature structural proteins are generated via cleavage by the host signal peptidase. The junction between NS2 and NS3 is autocatalytically cleaved by the NS2/NS3 protease, while the remaining four junctions are cleaved by the N-terminal serine protease domain of NS3 complexed with NS4A. The NS3 protein also contains the NTP-dependent helicase activity which unwinds duplex RNA during replication. The NS5B protein possesses RNA-dependent RNA polymerase (RDRP) activity, which is essential for viral replication. Unlike HBV or HIV, no DNA is involved in the replication of HCV.

U. S. Patent Publication (US 2005/0009737 A1) discloses that 1-(2-deoxy-2-fluoro-2-C-methyl-β-D-ribofuranosyecytosine (14) is a potent and selective anti-HCV agent. Previously known synthetic procedures (Schemes 1-3) for this compound are quite inefficient, with very low overall yields and are not amendable to large-scale.

Previously known methods for the preparation of (2′R)-2′-deoxy-2′-fluoro-2′-C-methyl nucleosides, and its analogues, from D-xylose, cytidine, or uridine employed DAST or Deoxofluor® for the key fluorination reaction. However, DAST and Deoxofluor® are expensive, hazardous for industrial synthesis, and provide often unreliable results. Therefore, these alkylaminosulfur trifluorides are not suitable for industrial production.

As a part of an effort to find better fluorination conditions, it has been discovered that opening of a cyclic sulfate with non-alkylaminosulfur trifluoride fluorinating agents is an excellent way to synthesize the anti-HCV nucleoside, (2′R)-2′-deoxy-2′-fluoro-2′-C-methylcytidine. In addition, it was discovered that this novel synthetic route can be adopted to other nucleosides including the anti-HCV nucleoside, D-2-deoxy-2-fluoro-cytidine (Devos, et al, U.S. Pat. No. 6,660,721), anti-HBV nucleosides, D and L-2′,3′-didehydro-2′,3′-dideoxy-2′-fluoro-nucleosides (Schinazi, et al, U.S. Pat. No. 6,348,587) (I and II, FIG. 3) as well as other 2′-substituted nucleosides such as D- and L-FMAU (Su, et al., J Med. Chem, 1986, 29, 151-154; Chu, et al., U.S. Pat. No. 6,512,107).

What is needed is a novel and cost effective process for the synthesis of 2′-C-alkyl-2′-deoxy-2′-substituted-D-ribopyranosyl nucleosides that have activity against HCV.

SUMMARY OF INVENTION

The present invention as disclosed herein relates to various intermediates and synthetic methods for the preparation of compounds of general formulas [I] and [II],

wherein

-   -   X is halogen (F, Cl, Br),     -   Y is N or CH,     -   Z is halogen, OH, OR′ SH, SR′, NH₂, NHR′, or R′     -   R² is alkyl of C₁-C₃, vinyl, or ethynyl;     -   R^(3′) and R^(5′) can be same or different H, alkyl, aralkyl,         acyl, cyclic acetal such as 2′,3′-O-isopropylidene or         2′,3-O-benzylidene, or 2′,3′-cyclic carbonate.     -   R², R⁴, and R⁵ are independently H, halogen including F, Cl, Br,         I, OH, OR′, SH, SR′, N₃, NH₂, NHR′, NR′₂, NHC(O)OR′, lower alkyl         of C₁-C₆, halogenated (F, Cl, Br, I) lower alkyl of C₁-C₆ such         as CF₃ and CH₂CH₂F, lower alkenyl of C₂-C₆ such as CH═CH₂,         halogenated (F, Cl, Br, I) lower alkenyl of C₂-C₆ such as         CH═CHCl, CH═CHBr and CH═CHI, lower alkynyl of C₂-C₆ such as         C═CH, halogenated (F, Cl, Br, I) lower alkynyl of C₂-C₆, lower         alkoxy of C₁-C₆ such as CH₂OH and CH₂CH₂OH, halogenated (F, Cl,         Br, I) lower alkoxy of C₁C₆, CO₂H, CO₂R′, CONH₂, CONHR′, CONR′₂,         CH═CHCO₂H, CH═CHCO₂R′; and,     -   R′ is an optionally substituted alkyl or acyl of C₁-C₁₂         (particularly when the alkyl is an amino acid residue),         cycloalkyl, optionally substituted alkynyl of C₂-C₆, optionally         substituted lower alkenyl of C₂-C₆, or optionally substituted         acyl.

DETAILED DESCRIPTION

Presently no preventive means against Flaviviridae, including hepatitis C virus (HCV), Dengue virus (DENV), West Nile virus (WNV) or Yellow Fever virus (YFV), infection is available. The only approved therapies are for treatment of HCV infection with alpha interferon alone or in combination with the nucleoside ribavirin, but the therapeutic value of these treatments has been compromised largely due to adverse effects. It was recently discovered that a group of nucleosides, including 2′-deoxy-2′-fluoro-2′-C-methylcytidine, exhibit potent and selective activity against replication of HCV in a replicon system. However, the difficulty of chemical synthesis of this and analogous nucleosides impedes further biophysical, biochemical, pharmacological evaluations mandatory for development of clinical drugs for treatment of Flaviviridae infection.

The present invention provides an efficient preparation of nucleosides and intermediates containing the 2-deoxy-2-fluoro-2-C-methyl-D-ribofuranosyl moiety.

Definitions

The term “independently” is used herein to indicate that the variable, which is independently applied, varies independently from application to application. Thus, in a compound such as R^(a)XYR^(a), wherein R^(a) is “independently carbon or nitrogen”, both R^(a) can be carbon, both R^(a) can be nitrogen, or one R^(a) can be carbon and the other R^(a) nitrogen.

As used herein, the terms “enantiomerically pure” or “enantiomerically enriched” refers to a nucleoside composition that comprises at least approximately 95%, and preferably approximately 97%, 98%, 99% or 100% of a single enantiomer of that nucleoside.

As used herein, the term “substantially free of” or “substantially in the absence of” refers to a nucleoside composition that includes at least 85 or 90% by weight, preferably 95% to 98% by weight, and even more preferably 99% to 100% by weight, of the designated enantiomer of that nucleoside. In a preferred embodiment, in the methods and compounds of this invention, the compounds are substantially free of enantiomers

The term “alkyl,” as used herein, unless otherwise specified, refers to a saturated straight or branched hydrocarbon chain of typically C₁ to C₁₀, and specifically includes methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, pentyl, isopentyl, neopentyl, hexyl, isohexyl, cyclohexyl, cyclohexylmethyl, 3-methylpentyl, 2,2-dimethylbutyl, and 2,3-dimethylbutyl, and the like. The term includes both substituted and unsubstituted alkyl groups. Alkyl groups can be optionally substituted with one or more moieties selected from the group consisting of hydroxyl, amino, alkylamino, arylamino, alkoxy, aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid, phosphate, or phosphonate. One or more of the hydrogen atoms attached to carbon atom on alkyl may be replaced by one or more halogen atoms, e.g. fluorine or chlorine or both, such as trifluoromethyl, difluoromethyl, fluorochloromethyl, and the like. The hydrocarbon chain may also be interrupted by a heteroatom, such as N, O or S.

The term “lower alkyl,” as used herein, and unless otherwise specified, refers to a C₁ to C₄ saturated straight or branched alkyl group, including both substituted and unsubstituted forms as defined above. Unless otherwise specifically stated in this application, when alkyl is a suitable moiety, lower alkyl is preferred. Similarly, when alkyl or lower alkyl is a suitable moiety, unsubstituted alkyl or lower alkyl is preferred.

The term “cycloalkyl”, as used herein, unless otherwise specified, refers to a saturated hydrocarbon ring having 3-8 carbon atoms, preferably, 3-6 carbon atoms, such as cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl. The cycloalkyl group may also be substituted on the ring by an alkyl group, such as cyclopropylmethyl and the like.

The terms “alkylamino” or “arylamino” refer to an amino group that has one or two alkyl or aryl substituents, respectively.

The term “protected,” as used herein and unless otherwise defined, refers to a group that is added to an oxygen, nitrogen, or phosphorus atom to prevent its further reaction or for other purposes. A wide variety of oxygen and nitrogen protecting groups are known to those skilled in the art of organic synthesis. Non-limiting examples include: C(O)-alkyl, C(O)Ph, C(O)aryl, CH₃, CH₂-alkyl, CH₂-alkenyl, CH₂Ph, CH₂-aryl, CH₂O-alkyl, CH₂O-aryl, SO₂-alkyl, SO₂-aryl, tert-butyldimethylsilyl, tert-butyldiphenylsilyl, and 1,3-(1,1,3,3-tetraisopropyldisiloxanylidene).

The term “aryl,” as used herein, and unless otherwise specified, refers to phenyl, biphenyl, or naphthyl, and preferably phenyl. The term includes both substituted and unsubstituted moieties. The aryl group can be substituted with one or more substituents, including, but not limited to hydroxyl, halo, amino, alkylamino, arylamino, alkoxy, aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid, phosphate, or phosphonate, either unprotected, or protected as necessary, as known to those skilled in the art, for example, as taught in T.W. Greene and P.G.M. Wuts, “Protective Groups in Organic Synthesis,” 3rd ed., John Wiley & Sons, 1999.

The terms “alkaryl” or “alkylaryl” refer to an alkyl group with an aryl substituent. The terms “aralkyl” or “arylalkyl” refer to an aryl group with an alkyl substituent, as for example, benzyl.

The term “halo,” as used herein, includes chloro, bromo, iodo and fluoro.

The term “acyl ester” or “O-linked ester” refers to a carboxylic acid ester of the formula C(O)R′ in which the non-carbonyl moiety of the ester group, R′, is a straight or branched alkyl, or cycloalkyl or lower alkyl, alkoxyalkyl including methoxymethyl, aralkyl including benzyl, aryloxyalkyl such as phenoxymethyl, aryl including phenyl optionally substituted with halogen (F, Cl, Br, I), C₁ to C₄ alkyl or C₁ to C₄ alkoxy, sulfonate esters such as alkyl or aralkyl sulphonyl including methanesulfonyl, the mono, di or triphosphate ester, trityl or monomethoxytrityl, substituted benzyl, trialkylsilyl (e.g. dimethyl-t-butyl silyl) or diphenylmethylsilyl. Aryl groups in the esters optimally include a phenyl group.

The term “acyl” refers to a group of the formula R″C(O)—, wherein R″ is a straight or branched alkyl, or cycloalkyl, amino acid, aryl including phenyl, alkylaryl, aralkyl including benzyl, alkoxyalkyl including methoxymethyl, aryloxyalkyl such as phenoxymethyl; or substituted alkyl (including lower alkyl), aryl including phenyl optionally substituted with chloro, bromo, fluoro, iodo, C₁ to C₄ alkyl or C₁ to C₄ alkoxy, sulfonate esters such as alkyl or aralkyl sulphonyl including methanesulfonyl, the mono, di or triphosphate ester, trityl or monomethoxy-trityl, substituted benzyl, alkaryl, aralkyl including benzyl, alkoxyalkyl including methoxymethyl, aryloxyalkyl such as phenoxymethyl. Aryl groups in the esters optimally comprise a phenyl group. In particular, acyl groups include acetyl, trifluoroacetyl, methylacetyl, cyclopropylacetyl, cyclopropyl carboxy, propionyl, butyryl, isobutyryl, hexanoyl, heptanoyl, octanoyl, neo-heptanoyl, phenylacetyl, 2-acetoxy-2-phenylacetyl, diphenylacetyl, α-methoxy-α-trifluoromethyl-phenylacetyl, bromoacetyl, 2-nitro-benzeneacetyl, 4-chloro-benzeneacetyl, 2-chloro-2,2-diphenylacetyl, 2-chloro-2-phenylacetyl, trimethylacetyl, chlorodifluoroacetyl, perfluoroacetyl, fluoroacetyl, bromodifluoroacetyl, methoxyacetyl, 2-thiopheneacetyl, chlorosulfonylacetyl, 3-methoxyphenylacetyl, phenoxyacetyl, tert-butylacetyl, trichloroacetyl, monochloro-acetyl, dichloroacetyl, 7H-dodecafluoro-heptanoyl, perfluoro-heptanoyl, 7H-dodeca-fluoroheptanoyl, 7-chlorododecafluoro-heptanoyl, 7-chloro-dodecafluoro-heptanoyl, 7H-dodecafluoroheptanoyl, 7H-dodeca-fluoroheptanoyl, nona-fluoro-3,6-dioxa-heptanoyl, nonafluoro-3,6-dioxaheptanoyl, perfluoroheptanoyl, methoxybenzoyl, methyl 3-amino-5-phenylthiophene-2-carboxyl, 3,6-dichloro-2-methoxy-benzoyl, 4-(1,1,2,2-tetrafluoro-ethoxy)-benzoyl, 2-bromo-propionyl, omega-aminocapryl, decanoyl, n-pentadecanoyl, stearyl, 3-cyclopentyl-propionyl, 1-benzene-carboxyl, O-acetylmandelyl, pivaloyl acetyl, 1-adamantane-carboxyl, cyclohexane-carboxyl, 2,6-pyridinedicarboxyl, cyclopropane-carboxyl, cyclobutane-carboxyl, perfluorocyclohexyl carboxyl, 4-methylbenzoyl, chloromethyl isoxazolyl carbonyl, perfluorocyclohexyl carboxyl, crotonyl, 1-methyl-1H-indazole-3-carbonyl, 2-propenyl, isovaleryl, 1-pyrrolidinecarbonyl, 4-phenylbenzoyl. When the term acyl is used, it is meant to be a specific and independent disclosure of acetyl, trifluoroacetyl, methylacetyl, cyclopropylacetyl, propionyl, butyryl, isobutyryl, hexanoyl, heptanoyl, octanoyl, neo-heptanoyl, phenylacetyl, diphenylacetyl, ct-trifluoromethyl-phenylacetyl, bromoacetyl, 4-chloro-benzeneacetyl, 2-chloro-2,2-diphenylacetyl, 2-chloro-2-phenylacetyl, trimethylacetyl, chlorodifluoroacetyl, perfluoroacetyl, fluoroacetyl, bromodifluoroacetyl, 2-thiopheneacetyl, tert-butylacetyl, trichloroacetyl, monochloro-acetyl, dichloroacetyl, methoxybenzoyl, 2-bromo-propionyl, decanoyl, n-pentadecanoyl, stearyl, 3-cyclopentyl-propionyl, 1-benzene-carboxyl, pivaloyl acetyl, 1-adamantane-carboxyl, cyclohexane-carboxyl, 2,6-pyridinedicarboxyl, cyclopropane-carboxyl, cyclobutane-carboxyl, 4-methylbenzoyl, crotonyl, 1-methyl-1H-indazole-3-carbonyl, 2-propenyl, isovaleryl, 4-phenylbenzoyl.

The term “lower acyl” refers to an acyl group in which R″, above defined, is lower alkyl.

The term “natural nucleic base” and “modified nucleic base” refer to “purine” or “pyrimidine” bases as defined below.

The term “purine” or “pyrimidine” base includes, but is not limited to, adenine, N⁶-alkylpurines, N⁶-acylpurines (wherein acyl is C(O)(alkyl, aryl, alkylaryl, or arylalkyl), N⁶-benzylpurine, N⁶-halopurine, N⁶-vinylpurine, N⁶-acetylenic purine, N⁶-acyl purine, N⁶-hydroxyalkyl purine, N⁶-allylaminopurine, N⁶-thioallyl purine, N⁶-alkylpurines, N²-alkyl-6-thiopurines, thymine, cytosine, 5-fluorocytosine, 5-methylcytosine, 6-azapyrimidine, including 6-azacytosine, 2- and/or 4-mercaptopyrimidine, uracil, 5-halouracil, including 5-fluorouracil, C⁵-alkylpyrimidines, C⁵-benzylpyrimidines, C⁵-halopyrimidines, C⁵-vinylpyrimidine, C⁵-acetylenic pyrimidine, C⁵-acyl pyrimidine, N⁴-acetylcytosine, N⁴-benzoylcytosine, N⁴-alkyl pyrimidine, C⁵-hydroxyalkyl purine, C⁵-amidopyrimidine, C⁵-cyanopyrimidine, C⁵-iodopyrimidine, C⁶-iodo-pyrimidine, C⁵—Br-vinyl pyrimidine, C⁶—Br-vinyl pyrimidine, C⁵-nitropyrimidine, C⁵-amino-pyrimidine, N²-alkylpurines, N²-alkyl-6-thiopurines, 5-azacytidinyl, 5-azauracilyl, triazolopyridinyl, imidazolopyridinyl, pyrrolopyrimidinyl, and pyrazolopyrimidinyl. Purine bases include, but are not limited to, guanine, adenine, hypoxanthine, 2,6-diaminopurine, and 6-chloropurine. Functional oxygen and nitrogen groups on the base can be protected as necessary or desired. Suitable protecting groups are well known to those skilled in the art, and include trimethylsilyl, dimethylhexylsilyl, t-butyldimethylsilyl, and t-butyldiphenylsilyl, trityl, alkyl groups, and acyl groups such as acetyl and propionyl, methanesulfonyl, and p-toluenesulfonyl.

The term “amino acid” includes naturally occurring and synthetic α, β γ or δ amino acids, and includes but is not limited to, amino acids found in proteins, i.e. glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline, serine, threonine, cysteine, tyrosine, asparagine, glutamine, aspartate, glutamate, lysine, arginine and histidine. In a preferred embodiment, the amino acid is in the L-configuration. Alternatively, the amino acid can be a derivative of alanyl, valinyl, leucinyl, isoleucinyl, prolinyl, phenylalaninyl, tryptophanyl, methioninyl, glycinyl, serinyl, threoninyl, cysteinyl, tyrosinyl, asparaginyl, glutaminyl, aspartoyl, glutaroyl, lysinyl, argininyl, histidinyl, β-alanyl, β-valinyl, β-leucinyl, β-isoleucinyl, β-prolinyl, β-phenylalaninyl, β-tryptophanyl, β-methioninyl, β-glycinyl, β-serinyl, β-threoninyl, β-cysteinyl, β-tyrosinyl, β-asparaginyl, β-glutaminyl, β-aspartoyl, β-glutaroyl, β-lysinyl, β-argininyl or β-histidinyl. When the term amino acid is used, it is considered to be a specific and independent disclosure of each of the esters of α, β γ or δ glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline, serine, threonine, cysteine, tyrosine, asparagine, glutamine, aspartate, glutamate, lysine, arginine and histidine in the D and L-configurations.

The term “pharmaceutically acceptable salt or prodrug” is used throughput the specification to describe any pharmaceutically acceptable form (such as an ester, phosphate ester, salt of an ester or a related group) of a compound which, upon administration to a patient, provides the active compound. Pharmaceutically acceptable salts include those derived from pharmaceutically acceptable inorganic or organic bases and acids. Suitable salts include those derived from alkali metals such as potassium and sodium, alkaline earth metals such as calcium and magnesium, among numerous other acids well known in the pharmaceutical art. Pharmaceutically acceptable salts may also be acid addition salts when formed with a nitrogen atom. Such salts are derived from pharmaceutically acceptable inorganic or organic acids, such as hydrochloric, sulfuric, phosphoric, acetic, citric, tartaric, and the like. Pharmaceutically acceptable prodrugs refer to a compound that is metabolized, for example hydrolyzed or oxidized, in the host to form the compound of the present invention. Typical examples of prodrugs include compounds that have biologically labile protecting groups on a functional moiety of the active compound. Prodrugs include compounds that can be oxidized, reduced, aminated, deaminated, hydroxylated, dehydroxylated, hydrolyzed, dehydrolyzed, alkylated, dealkylated, acylated, deacylated, phosphorylated, dephosphorylated to produce the active compound.

Applicants have developed a novel, practical and efficient process for the synthesis of 2-C-alkyl-2-deoxy-2-substituted-D-ribofuranose derivatives, the key intermediates to 14 (Scheme 1) and derivatives and analogues thereof using or without using chiral catalysts. The key step in the synthesis of 14 is asymmetric conversion of 41 to 42 using chiral catalysts (Scheme 4). The previous disclosed synthesis of 42 required Sharpless AD catalysts, such as dihydroquinidine (DHQD) and derivatives. The present invention as disclosed herein relates to the stereoselective preparation of 41 to 42 using osmium, osmate or permanganate without chiral catalysts. The applicants in this present invention also develop a practical and efficient process for the synthesis of 49 from 42 by using the nucleophilic opening of the cyclic sulfate 50 (Scheme 6) in highly stereospecific and regioselective manner. The procedure depicted in Schemes 4, 5 and 6 are the current method of choice for preparative synthesis of 14 and related derivatives.

I. Preparation of the Compounds

(i) Synthesis of the Cyclic Sulfite (Ma) and Cyclic Sulfate (IIIb)

This invention relates to the process for the preparation of the 2′-F-nucleosides and other 2′-substituted nucleosides of the general formula IB and IB-L- by using the nucleophilic opening of the cyclic sulfite, Ma (X═SO), sulfate, IIIb (X═SO₂), of the formula, III in highly stereospecific and regioselective manner, via the lactones of the formula, IV.

Wherein the formula IB, IB-L, III, IV has following specifications:

-   -   R′ is independently a lower alkyl (C₁-C₆) including, but not         limited to methyl, ethyl, optionally substituted phenyl,         optionally substituted benzyl; alternatively R′ is a part of         cyclic alkylene including ethylene (—CH₂CH₂—), or trimethylene         (—CH₂CH₂CH₂—) forming cyclic pentyl or cyclic hexanyl group;     -   R², R³ are independently hydrogen, a lower alkyl (C₁-C₆)         including, but not limited to methyl, hydroxymethyl,         methoxymethyl, halomethyl including, but not limited to         fluoromethyl, ethyl, propyl, optionally substituted ethenyl         including, but not limited to vinyl, halovinyl (F—CH═C),         optionally substituted ethynyl including, but not limited to         haloethynyl (F—C≡C), optionally substituted allyl including, but         not limited to haloallyl (FHC═CH—CH₂—);     -   R⁴ is independently hydrogen, aryl including, but not limited to         phenyl, aryl alkyl including, but not limited to benzyl, lower         alkyl including, but not limited to, methyl, ethyl, propyl. Nu         is halogen (F, Cl, Br), N₃, CN, NO₃, CF₃, OR or NR where R is         acyl including, but not limited to acetyl, benzoyl, arylalkyl         including but not limited to benzyl, lower alkyl including, but         not limited to, methyl, ethyl, propyl, CH₂R where R is hydrogen,         lower alkyl including, but not limited to, methyl, ethyl,         propyl;     -   X is SO₂, SO, or CO; and     -   B is a natural or modified nucleic base.

In one embodiment, formula, IB is:

wherein,

-   -   R², R³ are independently hydrogen, a lower alkyl (C₁-C₆)         including, but not limited to methyl, hydroxymethyl,         methoxymethyl, halomethyl including, but not limited to         fluoromethyl, ethyl, propyl, optionally substituted ethenyl         including, but not limited to vinyl, halovinyl (F—CH═C),         optionally substituted ethnyl including, but not limited to         haloethnyl (F—C═C), optionally substituted allyl including, but         not limited to haloallyl (FHC═CH—CH₂—);     -   B is a natural or modified nucleic base.

The present invention as disclosed herein relates to processes for the synthesis of a compound, 2-alkyl-4,5-di-O-protected-2,3-dihydroxy-pentanoic-acid ester of the following general formula 42B, which is the important intermediate in the synthesis of anti-HCV nucleosides of general formulas [I] and [II] (below).

wherein R′, R″=isopropylidene, benzylidene or cyclohexylidene or a like, or a part of cyclic group including ethylene (—CH₂CH₂—), or trimethylene (—CH₂CH₂CH₂—) forming cyclopentyl or cyclohexanyl group, respectively; R′ and R″ can be independently lower alkyl of C₁-C₆, or aryl of C₆-C₂₀), benzyl and other optionally substituted benzyl, trialkylsilyl, t-butyl-dialkylsyl, t-butyldiphenylsilyl, TIPDS, THP, MOM, MEM and other optionally ether protecting groups; or H, acetyl, benzoyl and other optionally substituted acyl (R′ and R″ are —C(O)—R, wherein R can be lower alkyl of C₁-C₆, or aryl of C₆-C₂₀, benzyl or other optionally substituted benzyl);

R₁, R₂ are independently hydrogen, aryl (C₆-C₂₀) and a lower alkyl (C₁-C₆) including methyl, hydroxymethyl, methoxymethyl, halomethyl including fluoromethyl, ethyl, propyl, optionally substituted ethenyl including vinyl, halovinyl (F—CH═C), optionally substituted ethynyl including haloethynyl (F—C≡C), optionally substituted allyl including haloallyl (FHC═CH—CH₂—); and

R₃ is independently hydrogen, aryl including phenyl, aryl alkyl including, but not limited to benzyl, lower alkyl (C₁₋₆) including methyl, ethyl, or propyl.

The invention as disclosed herein also relates to processes for making compounds of the following general formula 49B, which are prepared from 2-alkyl-4,5-di-O-protected-2,3-dihydroxy-pentanoic-acid ester derivatives of general formula [42B].

wherein R³ and R⁵ can be independently H, CH₃, Ac, Bz, pivaloyl, or 4-nitrobenzoyl, 3-nitrobenzoyl, 2-nitrobenzoyl, 4-chlorobenzoyl, 3-chlorobenzoyl, 2-chlorobenzoyl, 4-methylbenzoyl, 3-methylbenzoyl, 2-methylbenzoyl, para-phenylbenzoyl, and other optionally substituted acyl (R³ and R⁵ are —C(O)—R, R can be independently lower alkyl of C₁-C₆, or aryl of C₆-C₂₀), benzyl, 4-methoxybenzyl and other optionally substituted benzyl (R³ and R⁵ can be independently aryl of C₆-C₂₀), trityl, trialkylsilyl, t-butyl-dialkylsyl, t-butyldiphenylsilyl, TIPDS, THP, MOM, MEM and other optionally ether protecting groups (R³ and R⁵ can be independently alkyl of C₁-C₁₀), or R³ and R⁵ are linked through —SiR₂—O—SiR₂— or —SiR₂—, wherein R is a lower alkyl group such as Me, Et, n-Pr or i-Pr.

wherein

-   -   X is halogen (F, Cl, Br),     -   Y is N or CH,     -   Z is, halogen, OH, OR′, SH, SR′, NW, NHR′, or R′     -   R^(2′) is alkyl of C₁-C₃, vinyl, or ethynyl     -   R^(3′) and R^(5′) can be same or different H, alkyl, aralkyl,         acyl, cyclic acetal such as 2′,3′-O-isopropylidene or         2′,3-O-benzylidene, or 2′,3′-cyclic carbonate.     -   R², R⁴, R⁵ and R⁶ are independently H, halogen including F, Cl,         Br, I, OH, OR′, SH, SR′, N₃, NW, NHR′, NR″, NHC(O)OR′, lower         alkyl of C₁-C₆, halogenated (F, Cl, Br, I) lower alkyl of C₁-C₆         such as CF₃ and CH₂CH₂F, lower alkenyl of C₂-C₆ such as CH═CH₂,         halogenated (F, Cl, Br, I) lower alkenyl of C₂-C₆ such as         CH═CHCl, CH═CHBr and CH═CHI, lower alkynyl of C₂-C₆ such as         C═CH, halogenated (F, Cl, Br, I) lower alkynyl of C₂-C₆, lower         alkoxy of C₁-C₆ such as CH₂OH and CH₂CH₂OH, halogenated (F, Cl,         Br, I) lower alkoxy of C₁-C₆, CO₂H, CO₂R′, CONH₂, CONHR′,         CONR′₂, CH═CHCO₂H, CH═CHCO₂R′; and,     -   R′ and R″ are the same or different and are optionally         substituted alkyl of C₁-C₁₂ (particularly when the alkyl is an         amino acid residue), cycloalkyl, optionally substituted alkynyl         of C₂-C₆, optionally substituted lower alkenyl of C₂-C₆, or         optionally substituted acyl.

The reaction of the cyclic sulfate ester, 50 (Scheme 6) with tetraethylammonium fluoride or tetramethylammonium fluoride 51 (Scheme 6) quantitatively generated the fluorinated sulfate, in highly stereospecific and regioselective manner. Following acid catalyzed cyclization afforded the 2-fluoro-2-C-methyl-γ-ribonolactone, 53 in high yield. The present invention is based on this discovery and provides a process for the preparation of the 2′-deoxy-2′-substituted nucleosides, I and II, using the reactions described herein.

(2S,3R,4R)-4,5-O-alkylidene-2-dimethyl-2,3,4,5-tetrahydroxy-2-methy-1-pentanoic acid ethyl ester (42B), can be prepared by asymmetric dihydroxylation (AD) or stereoselective dihydroxylation of the Wittig product 41 with or without chiral catalysts. Wittig product 41, in turn, can be prepared readily from the protected (R) glyceraldehyde (Schemes 7, 8), where R¹ is independently a lower alkyl (C₁-C₆) including, but not limited to methyl, ethyl, optionally substituted phenyl, optionally substituted benzyl. Or R′ is a part of cyclic group including ethylene (—CH₂CH₂—), or trimethylene (—CH₂CH₂CH₂—) forming cyclopentyl or cyclohexanyl group, respectively. R², R³ are independently hydrogen, a lower alkyl (C₁-C₆) including, but not limited to methyl, hydroxymethyl, methoxymethyl, halomethyl including, but not limited to fluoromethyl, ethyl, propyl, optionally substituted ethenyl including, but not limited to vinyl, halovinyl (F—CH═C), optionally substituted ethynyl including, but not limited to haloethynyl (F—C≡C), optionally substituted allyl including, but not limited to haloallyl (FHC═CH—CH₂—); and R⁴ is acyl including, but not limited to acetyl, benzoyl, arylalkyl including but not limited to benzyl, lower alkyl (C₁₋₁₀) including, but not limited to, methyl, ethyl, propyl, CH₂R where R is hydrogen, lower alkyl (C₁₋₁₀) including, but not limited to, methyl, ethyl, propyl.

The diol (42B) can be converted to the cyclic sulfite (IIIa) by treatment with thionyl chloride (SOCl₂) in presence of an alkylamine such as triethylamine, diisopropyl ethylamine, or pyridine, which can then be oxidized using the oxidants selected from a first group consisting of RuCl₃, KMnO₄, and TEMPO or a combination of the first group and one of the second group consisting of NaIO₄, KIO₄, HIO₄, mCPBA, NaOCl, and oxone. The solvent of this step is selected from one or more of the group consisting of chloroform, methylene chloride, 1,2-dichloroethane, diethyl ether, tetrahydrofuran, benzene, and toluene, alone or in combination with water. (Gao Y et al J. Am. Chem. Soc. 1988, 110, 7538-7539, Berridge et al J. Org. Chem. 1990, 55, 1211-1217). It is also possible that the diol is directly converted to the cyclic sulfate (IIIb) by treatment with sulfurylchloride, or sulfuryl diimidazole. On the other hand, the diol 42B can be converted to the cyclic carbonate (IIIc) by treatment with carbonyl diimidazole or carbonyl dimethoxide (Scheme 8) (Chang, et al Tetrahedron Lett. 1996, 37, 3219-3222).

(ii) Synthesis of the Substituted 2-deoxy-D-ribono-γ-lactone, 53B

The cyclic sulfate (IIIb, Scheme 8) can be converted to the fluorinated sulfate ester of the formula, 51B (Scheme 9), in high yield and with high regioselectivity and stereospecificity, by treatment with tetraalkylammonium fluoride including, but not limited to tetramethylammonium fluoride (TMAF), tetraethylammonium fluoride (TEAF), or tetrabutylammomnium fluoride (TBAF), or tris(dimehtylamino)sulfur (trimethylsilyl)difluoride (TAS-F) (Fuentes J, et al Tetrahedron lett. 1998, 39, 7149-7152) in an aprotic polar solvent such as acetone, tetrahydrofuran, N,N-dimethylformamide, or acetonitrile (Scheme 9). Metal fluorides such as silver fluoride (AgF), potassium fluoride (KF), cesium fluoride (CsF), or rubidium fluoride (RbF), can be used alone or with catalytic amount of tetraalkylammonium fluoride, crown-ether, diglyme, or polyethylene glycol, or other phase transfer catalyst.

The cyclic sulfate (IIIb) can be converted to other 2-substituted sulfates of the formula 51B by treatment with NaBH₄, tetraalkylammonium chloride, tetraalkylammonium bromide, NaN₃ or LiN₃, NH₄OR, NH₄SCN, CF₃I-tetrakis(dimethylamino)-ethylene (TDAE), and tetraalkylammonium nitrate (Gao et al J. Am. Chem. Soc. 1988, 110, 7538-7539), KCN, LiCu(R)₂ where R is methyl, ethyl, ethylenyl, or ethnyl. Similarly, the cyclicsulfite (Ma) can be converted to the substituted ester 52B (Chang et al. Tetrahedron Len. 1996, 37, 3219-3222). Then compounds of the formula 51B and 52B can be converted to the substituted lactones of the formula 53B by treatment with an acid in H₂O-containing organic solvent such as methanol, ethanol, or acetonitrile.

In Formula 53B, R², R³ is independently hydrogen, a lower alkyl (C₁-C₆) including, but not limited to methyl, hydroxymethyl, methoxymethyl, halomethyl including, but not limited to fluoromethyl, ethyl, propyl, optionally substituted ethenyl including, but not limited to vinyl, halovinyl (F—CH═C), optionally substituted ethynyl including, but not limited to haloethynyl (F—C≡C), optionally substituted allyl including, but not limited to haloallyl (FHC═CH—CH₂—). Nu is halogen (F, Cl, Br), N₃, CN, NO₃, CF₃, SCN, OR or NR₂ where R is acyl including, but not limited to acetyl, benzoyl, arylalkyl including but not limited to benzyl, lower alkyl (C₁₋₁₀) including, but not limited to methyl, ethyl, propyl, CH₂R where R is hydrogen, lower alkyl (C₁₋₁₀) including, but not limited to methyl, ethyl, propyl.

(iii) The Protection of the D-ribono-γ-lactone, 53B

53B can be selectively protected with appropriate protection agents to the 5-protected lactones of the formula 53C with an appropriate base in an appropriate solvent. The protecting group includes, but is not limited to the following: trityl, t-butyldimethylsilyl, t-butyldiphenylsilyl, benzyloxymethyl, benzoyl, toluoyl, 4-phenyl benzoyl, 2-, 3-, or 4-nitrobenzoyl, 2-, 3-, or 4-chlorobenzoyl, other substituted benzoyl. The base includes, but is not limited to the following: imidazole, pyridine, 4-(dimethylamino)pyridine, triethytlamine, diisopropylethylamine, 1,4-diazabicyclo[2,2,2]-octane. The solvent includes, but is not limited to the following: pyridine, dichloromethane, chloroform, 1,2-dichloroethane, tetrahydrofuran.

Alternatively, the lactone 53B can be fully protected with appropriate protection agents with an appropriate base in an appropriate solvent. The protecting group (R⁵, R⁶) includes, but is not limited to the following: methoxymethyl, methoxyethyl, benzyloxymethyl, ethoxymethyl, trityl, triethylsilyl, t-butyldimethylsilyl, t-butyldiphenylsilyl, acyl including acetyl, pivaloyl, benzoyl, toluoyl, 4-phenyl benzoyl, 2-, 3-, or 4-nitrobenzoyl, 2-, 3-, or 4-chlorobenzoyl, other substituted benzoyl. The base includes, but is not limited to the following list: imidazole, pyridine, 4-(dimethylamino)pyridine, triethytlamine, diisopropylethylamine, 1,4-diazabicyclo[2,2,2]octane. The solvent includes, but is not limited to pyridine, dichloromethane, chloroform, 1,2-dichloroethane, tetrahydrofuran (Scheme 10).

(ii) Complexation Directed β-glycosylation

Coupling of 2-deoxy-2-fluoro-2-C-methyl-ribofuranoside (54: Nu=F, R³=Me, R⁵═R⁶=pivaloyl) with silylated N⁴-benzoylcytosine in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) in CHCl₃ gave a mixture of α/β-anomers with a ratio of 2/1 in favor of α-isomer. However, β-anomer was obtained as major product (α/β=1/4.9) in the same reaction catalyzed by SnCl₄ under similar conditions. Possible mechanisms are proposed in Scheme 10A (R⁵ and R⁶ are O-protecting groups that can be acyl or silyl or alkyl or aralkyl with C₁₋₂₀). Treatment of 54 with silylated N⁴-benzoylcytosine in the presence of TMSOTf in CHCl₃ formed an oxonium intermediate 54-i. Silylated base could attack 54-1 from up-side to give β-anomer 55B or from bottom to provide α-anomer 55B-alpha. Because of stereohinderance at up-side caused by 2-methyl group, silylated base attacked intermediate 54-i mainly from bottom (less stereohindered side) to afford a mixture of α/β-anomers with a ratio of 2/1 in favor of α-anomer. While treatment of 54 with silylated N⁴-benzoylcytosine in the presence of SnCl₄, a complex 54-ii was formed instead of oxonium 54-i. Silyated N⁴-benzoylcytosine attacked 54-ii from less stereohindered up-side to give a mixture of α/β-anomers with a ratio of 1/5 in favor of β-anomer.

Compound 54 can be made from the protected lactone of the formula, 49B, which can be reduced with DIBAL-H or lithium tri-tert-butoxyaluminum hydride and other hydride reducing agent to the lactol, which can then converted either to the acylate by acylation with acyl halide, or acyl anhydride, in presence of an appropriate base in an appropriate solvent. Acyl halide or acyl anhydride includes, but is not limited to the following list: acetic chloride, optionally substituted benzoyl chloride, acetic anhydride, optionally substituted benzoyl anhydride. The base includes, but is not limited to the following: imidazole, pyridine, 4-(dimethylamino)pyridine, triethytlamine, diisopropylethylamine, 1,4-diazabicyclo[2,2,2]octane. The solvent includes, but is not limited to the following list: pyridine, dichloromethane, chloroform, 1,2-dichloroethane, tetrahydrofuran.

(iii) Synthesis of the L-nucleosides, IB-L

The processes for the D-series of the formula I and II can be used for preparation of the L-nucleosides of the formula, IB-L from the (S)-glyceraldehydes (Scheme 11).

(iv) Synthesis of 2-alkyl-4,5-di-O-protected-2,3-dihydroxy-pentanoic Acid

Currently, the most preferable procedure for the synthesis of nucleosides of general structures I and II is the preparation of a derivative of the 2-deoxy-2-fluoro-2-C-methyl-D-ribofuranosyl moiety of I and II as shown in Scheme 4, Scheme 5 and Scheme 6, above through (i) synthesis of the intermediate, derivatives of 2-alkyl-4,5-di-O-protected-2,3-dihydroxy-pentanoic-acid ester of general structure I, (ii) conversion of 42B into the 3,5-protected 2-deoxy-2-fluoro-2-C-methyl-D-ribono-γ-latone of general structure 49B, and (iii) conversion of 49B into purine and pyrimidine nucleosides of general structures of I and II. The key step in Scheme 4 is the stereoselective osmium catalyzed dihydroxylation of olefinic intermediate 41 into 42 in the presence of the expensive Sharpless AD catalyst. Instead of the Sharpless catalyst, if other chiral compounds such as L-quinidine are used, the reaction also goes smoothly giving the desired 42. Kishi et al. have proposed that in Osa₄ dihydroxylation of allylic alcohol derivatives (esters, ethers, acetals or ketals), the major course of reaction would occur on the face of the olefinic bond opposite to that of the preexisting hydroxyl or alkoxyl group, (Tetrahedron Lett, 1983, 24, 3943). Some examples are shown in Scheme 12 (Tetrahedron Lett, 1983, 24, 3947). In every case, the major product arose from addition of OSO₄ from the anti side of the oxygen on the neighboring secondary carbon. However, stereoselectivity is not high enough for preparative synthesis.

Encouraged by Kishi's rule, which presents that the stereochemistry is formulated as arising from the preferential approach of osmium tetroxide to occur on the face of the olefinic bond opposite to that of the preexisting hydroxyl or alkoxyl group, dihydroxylations of 41 under the original conditions but without any chiral catalysts, including Sharpless AD catalyst, were conducted. Dihydroxylation of 41 using Ke₃Fe(CN)₆/K₂OsO₂(OH)₄/K₂CO₃ system without chiral catalysts gives the product in 77% yield, which product is a 5:1 mixture of isomers with the predominant isomer being the desired 42. The reaction of olefin 41 with OsO₄ using N-methylmorpholine N-oxide (NMO) as the oxidant without chiral catalysts gave a 5:1 mixture of 42 and its isomer in 79% yield. Most surprisingly, when t-butylhydroperoxide (TBHP) is used as oxidant in the presence of catalytic amount of OSO₄ in acetone and ammonium acetate as buffer (the reagent combination was used in the synthesis of alditols by Masamune and Sharpless (J. Org. Chem, 1982, 47, 1373)), the crystalline product isolated is the virtually pure desired 42. This procedure is therefore far superior to the OSO₄/NMO and Fe(CN)₆ ³⁻ methods. At 10 mmolar scale, the desired diol 42 is formed exclusively, and is isolated in 87% yield. No contamination by the other isomer was detected in this product by vigorous NMR analyses.

It is well known that in OSO₄ oxidation the intermediate is cyclic osmate V (below) (Criegee, Liebigs Ann. Chem., 1936, 522, 75). cis-Dihydroxylation of olefins with potassium permanganate in alkaline media has been known for quite some time (Robinson and Robinson, J. Chem. Soc., 1925, 127, 1628), and this reaction appears to proceed through a cyclic ester VI. Thus attempts at permanganate dihydroxylation have been performed.

Previous reports have indicated that permanganate dihydroxylation of olefins in acid or neutral conditions causes over-oxidation of the initial diol products with concomitant production of ketones and carboxylates. Only in alkaline conditions further oxidation of the diol products can be decelerated. As 41 is a carboxylic ester the reaction cannot be done in aqueous alkali. Hazra et al. (J. Chem. Soc. Perkin Trans. I, 1994, 1667) describes successful dihydroxylation of highly substituted olefins to the corresponding diols using tetradecyltrimethylammonium permanganate (TDTAP) in a mixture of t-BuOH, dichloromethane and water in the presence of 0.1 equivalent of KOH. Application of this method to dihydroxylation of 41 results in rapid formation (within 10 minutes at room temperature) of a mixture of 42 and its diastereomer in an 8:1 ratio, which is isolated in 71% yield. Oxidation occurs much faster in similar reactions without KOH, but the yield of 42 is not improved.

Mukaiyama et al. (Chem. Lett., 1983, 173) disclosed dihydroxylation of olefins with KMnO₄ and 18-crown-6 ether in dichloromethane at −40° C. Attempts at dihydroxylation of 41 under Mukaiyama's conditions but at different temperatures offer a 6:1 mixture of 42 and its diastereomer in 50% yield at −40° C. and the same mixture in 94% yield at −10° C.

Surprisingly, in contrast to the teaching of the prior of art which discloses that oxidation of a double bond with KMnO₄ proceeds via diol wherein the resultant diol is rapidly oxidized further without the presence of base, diol 42 was found to be isolable when the corresponding 41 is treated with KMnO₄ without added alkali and crown ether. In pure t-butanol, oxidation does not proceed even at room temperature conditions for two days. Addition of water to the mixture promotes the reaction. It is found that the more water in the reaction media the faster the reaction proceeds with poor selectivity of 42 production; the less water the slower the reaction but improved selectivity. In any case, the yield is rather poor due to further oxidation.

Most surprisingly, and in contradiction to the prior art, treatment of 41 with KMnO₄ in acetone is found to give a 10:1 mixture in quantitative yield, the desired 42 being the major component. The stereoselectivity is found to be improved by performing the reaction in a mixture of acetone and pyridine.

The following Examples are set forth to aid in an understanding of the invention. This section is not intended to, and should not be interpreted to, limit in any way the invention set forth in the claims which follow thereafter.

EXAMPLES Example 1 (2S, 3R, 4R)-4,5-O-isopropylidene-2,3-O-sulfuryl-2,3,4,5-tetrahydroxy-2-methyl-pentanoic Acid Ethyl Ester (IIIb, R¹═CH₃, R²═H, R³═CH₃)

To a solution of (2S, 3R, 4R)-4,5-O-isopropylidene-2,3,4,5-tetrahydroxy-2-methyl-pentanoic acid ethyl ester (R¹═CH₃, R²═H, R³═CH₃) (2.0 g, 8.06 mmol) in anhydrous methylene chloride (40 mL) containing triethyl amine (3.4 mL) was added at 0° C. thionyl chloride (0.88 mL, 12.08 mmol) dropwise over 10 min. The resulting reaction mixture was stirred at 0° C. for 10 min, diluted with cold ether (100 mL), washed with water (50 mL×2) and brine (50 mL×2), dried with sodium sulfate, and concentrated to give a residue (IIIa, R¹═CH₃, R²═H, R³═CH₃) which was dissolved in acetonitrile-tetrachloromethane (10:10 mL). To the obtained solution was added at room temperature sodium periodate (2.58 g, 12.06 mmol), ruthenium trichloride (16 mg, 0.077 mmol), and water (14 mL) subsequently. The resulting reaction mixture was stirred at room temperature for 10 min, diluted ether (100 mL), washed with water (50 mL×2), saturated sodium bicarbonate solution (50 mL×2), and brine (50 mL×2), dried with sodium sulfate, concentrated, and co-evaporated with toluene (30 mL×3) to a syrupy residue, the sulfate IIIb (2.23 g, 89%) which was used for the next reaction without further purification. ¹H NMR (CDCl₃) δ (ppm) 5.04 (d, 1H, J=9.6 Hz, H-3), 4.37 (m, 1H, H-4), 4.29 (q, 2H, J=7.6 Hz, CH ₂CH₃), 4.17 (dd, 1H, J=5.6, 9.6 Hz, H-5), 4.05 (dd, 1H, J=3.2, 9.6 Hz, H-5′), 1.8 (s, 3H,CH₃-2), 1.38 (s, 3H, (CH ₃)₂C), 1.32 (t, 3H, J=6.8 Hz, CH₂CH ₃), 1.31 (s, 3H, (CH₃)₂C).

Example 2 Tetrabutylammonium Salt of (2R, 3S, 4R)-2-fluoro-4,5-O-isopropylidene-2-methyl-3-sulfooxy-3,4,5-trihydroxypentanoic Acid Ethyl Ester (51B, R¹═CH₃, R²═H, R³═CH₃, Nu=F, M⁺=tetrabutylammonium)

Method 1: To a solution of the sulfate IIIb from Example 1 (628 mg, 2.02 mmol) in anhydrous tetrahydrofuran was added at 0° C. tetrabutylammonium fluoride (1M in tetrahydrofuran, dried with 4 Å molecular sieves) dropwise over 5 min. The resulting reaction mixture was stirred at 0° C. for 20 min, another 2 m L of tetrabutylammonium fluoride (1M in tetrahydrofuran, dried with 4 Å molecular sieves, 3 mL) was added, and then the reaction mixture was stirred at 0° C. for 2 hours, then concentrated, and purified by silica gel column chromatography (EtOAc) to give to the fluorinated sulfate, as a syrup (350 mg, 38%). ¹H NMR (CDCl₃) δ (ppm) 4.66 (dd, 1H, J=9.6, 25.6 Hz, H-3), 4.48 (dd, 1H, J=5.2, 8.8 Hz, H-4), 4.20, 4.07 (2m, 4H, H-5, OCH ₂CH₃), 3.21 (m, 8H, N(CH ₂CH₂CH₂CH₃)₄), 1.69 (d, 3H, J=22.4 Hz, CH₃-2), 1.59 (m, 8H, N(CH₂CH ₂CH₂CH₃)₄), 1.39 (m, 8H, CH₂CH₂CH ₂CH₃)₄), 1.27-1.25 (m, 9H, OCH₂CH ₃, (CH ₃)₂C), 0.96 (t, 12H, J=6.8 Hz, CH₂CH₂CH₂CH ₃)₄.

Method 2: To a solution of the cyclic sulfate IIIb (480 mg, 1.55 mmol) in anhydrous tetrahydrofuran was added at 0° C. tetrabutylammonium fluoride (1M in tetrahydrofuran, neutralized with HF-pyridine, 3.1 mL) dropwise over 5 min. The resulting reaction mixture was stirred for 39 hours, concentrated, and purified by silica gel column chromatography (CH₂Cl₂:MeOH=10:1) to the fluorinated sulfate as a syrup (280 mg, 39%).

Example 3 2-Deoxy-2-fluoro-2-C-methyl-D-ribono-γ-lactone (53B, R²═H, R³═CH₃, Nu=F)

A mixture of the product of Example 2 (170 mg, 0.370 mmol), trifluoroacetic acid (0.8 mL), and water (2 mL) in acetonitrile (10 mL) was heated at 80° C. for 1.5 hours, diluted with ethyl acetate (15 mL), washed with water (10 mL) and saturated sodium bicarbonate solution (10 mL). The aqueous layer was saturated with NaCl and extracted with ethyl acetate (10 mL). The combined organic layer was dried with sodium sulfate, filtered, and concentrated to give a residue, which was purified by silica gel column chromatography (hexanes:ethyl acetate=1:1 to CH₂Cl₂:MeOH=20:1) to give the desired compound as a white solid (60 mg, 100%). ¹H NMR (CDCl₃) δ (ppm) 6.06 (d, 1H, J=6.8 Hz, HO-3), 5.16 (t, 1H, J=4.8 Hz, HO-5), 4.26 (m, 1H, H-4), 3.98 (ddd, 1H, J=7.2, 8.0, 23.2 Hz, H-3), 3.78 (ddd, 1H, J=2.0, 5.2, 12.8 Hz, H-5), 3.55 (ddd, 1H, J=4.4, 5.6, 12.4 Hz, H-5′), 1.48 (d, 3H, J=24 Hz, CH₃-2); ¹³C NMR (CDCl₃) S (ppm) 171.2 (d, J=21.2 Hz, C-1), 92.5 (d, J=177.5 Hz, C-2), 83.37 (C-4), 70.2 (d, J=15.9 Hz, C-3), 59.0 (C-5), 17.1 (d, J=25.0 Hz, CH₃—C-2).

Example 4 3,5-Di-O-benzoyl-2-deoxy-2-fluoro-2-C-methyl-D-ribono-γ-lactone (49B, R²═H, R³═CH₃, R⁵=Bz, R⁶=Bz, Nu=F)

The compound of Example 3 (60 mg, 0.16 mmol) was dissolved in anhydrous pyridine (1 mL) and benzoyl chloride (0.3 mL) was added. The resulting reaction mixture was stirred at room temperature for 20 min, water added (1 mL), stirred for 20 min, diluted with ethyl acetate (5 mL), washed with water (2 mL) and 1M HCl (2 mL×3), and dried with sodium sulfate. Upon filtration and concentration, the residue was purified by silica gel column chromatography (hexanes:ethyl acetate=10:1) to give 3,5-di-O-benzoyl-2-deoxy-2-fluoro-D-ribono-γ-lactone as a white solid (118 mg, 87%). ¹H NMR (CDCl₃) δ (ppm) 8.08 (m, 2H, aromatic), 7.99 (m, 2H, aromatic), 7.63 (m, 1H, aromatic), 7.58 (m, 1H, aromatic), 7.49 (m, 2H, aromatic), 7.43 (m, 2H, aromatic), 5.51 (dd, 1H, J=7.2, 17.6 Hz, H-3), 5.00 (m, 1H, H-4), 4.78 (dd, 1H, J=3.6, 12.8 Hz, H-5), 4.59 (dd, 1H, J=5.2, 12.8 Hz, H-5′), 1.75 (d, 3H, J=23.6 Hz, CH₃-2)

Example 5 Tetraethylammonium Salt of (2R, 3S, 4R)-4,5-dihydroxy-2-fluoro-4,5-O-isopropylidene-2-Acid Ethyl Ester (51B, R¹═CH₃, R²═H, R³═CH₃, Nu=F, M⁺=tetraethylammonium)

Method 1. To a solution of the sulfate IIIb (Scheme 9) (1.96 g, 6.32 mmol) in anhydrous N,N-dimethylformamide (20 mL) was added at 0° C. tetraethylammonium fluoride hydrate (1.39 g, 9.13 mmol) in one portion. The resulting reaction mixture was stirred for 30 min, concentrated, and co-evaporated with toluene to give a semi-solid (51b) (3.35 g, crude, proton NMR showed virtually one product). ¹H NMR (CDCl₃) δ (ppm) 4.61 (dd, 1H, J=9.2, 25.6 Hz, H-3), 4.51 (dd, 111, J=5.2, 9.2 Hz, H-4), 4.23-4.05 (m, 4H, H-5, OCH ₂CH₃), 3.32 (q, 811, J=7.2 Hz, N(CH ₂CH₃)₄), 1.69 (d, 3H, J=23. 2 Hz, CH₃-2), 1.31-1.24 (m, 21H, OCH₂CH ₃, (CH ₃)₂C, N(CH₂CH ₃)₄.

Method 2: To a solution of the sulfate IIIp (148 mg, 0.477 mmol) in anhydrous acetonitrile (2 mL) was added at 0° C. tetraethylammonium fluoride hydrate (107 mg, 0.717 mmol) in one portion. The resulting reaction mixture was stirred for 24 hours, concentrated, and co-evaporated with toluene to give a semi-solid (257 mg, crude, proton NMR showed virtually one product).

Example 6 Preparation of 1-(2-deoxy-2-fluoro-2-methyl-3,5-O-3,5 dipivaloyl-ribofuranosyly-N⁴-benzoylcytosine (11b, R⁵═R⁶=pivaloyl, R²═H, R³=Me)

To a solution of 49B, (Scheme 6) (Nu=F, R²—H, R³=Me, R⁵═R⁶=pivaloyl, 3.44 g, 10.36 mmol) in THF (70 mL) was added LiAl(t-BuO)₃H (13.47 mmol, 1M in THF, 13.47 mL) at −20° C. to −10° C. and the resulting solution was stirred at −10° C. to −15° C. for 2 h. To the solution was added an additional LiAl (t-BuO)₃H (1.35 mL, 1.35 mmol) and the solution was stirred at −10° C. for 1 h. Ice water (50 mL) was added. The mixture was extracted with EtOAc (200 mL), and the organic layer was washed with water, brine and dried (Na₂SO₄). Solvent was removed to give crude lactol which was dissolved in CH₂Cl₂ (50 mL). To the solution were added Et₃N (31.08 mmol, 4.24 mL), 4-dimethylaminopyridine (1 mmol, 122 mg) and trimethylacetyl chloride (20.7 mmol, 2.55 mL), and the mixture was stirred at room temperature for 16 h. Water (20 mL) was added, and the resulting mixture was stirred at room temperature for 10 min. EtOAc (200 mL) was added, and organic solution was washed with water, brine, and dried (Na₂SO₄). Solvent was removed and the residue was co-evaporated with toluene (2×20 mL) to give a crude intermediate (5, 6.74 g) for the next coupling reaction without purification.

A suspension of N⁴-benzoylcytosine (6.06 mmol, 1.30 g) and (NH₄)₂SO₄ (30 mmg) in HMDS (16.7 mL) was refluxed for 5 h, and the clear solution was concentrated to dryness under reduced pressure. The residue was dissolved in 1,2-dichloroethane (50 mL). To the solution were added crude 54 (1.96 g, Scheme 6) and SnCl₄ (1.42 mL, 12.12 mmol) at room temperature. The solution was refluxed for 24 h. and cooled to 0° C. To the solution were added NaHCO₃ (6.11 g, 72.72 mmol) and EtOAc (50 mL). To the mixture was added H₂O (2 mL) slowly, and the resulting mixture was stirred at room temperature for 20 min. Solid was removed by filtration. The organic solution was washed with water, brine and dried (Na₂SO₄). Solvent was removed to give syrup as crude mixture of β/α-anomers with a ratio of 4/1 in favor to β-isomer. The crude product was dissolved in MeOH (1 mL) at 50° C. To the solution was added hexanes (10 mL). The mixture was allowed to stay at room temperature for 1 h, then 0° C. for 2 h. Crystals were collected by filtration, washed with hexanes to give product 55, Scheme 6 (323 mg, 20.3% from 49). Mother liquor was concentrated to dryness and purified by column chromatography (20-50% EtOAc in hexanes) to give second crop of 55. H-NMR (CDCl₃): δ 8.82 (br s, 1H, NH), 8.10, 7.89, 7.62, 7.52 (m, 7H, H-5, H-6, 5Ph-H), 6.41 (d, J=18.4 Hz, 1H, H-1′), 5.10 (m, 1H, H-3′), 4.45 (d, J=9.6 Hz, 1H, H-4′), 4.36 (t, J=2.8 Hz, 2H, H-5′), 1.35 (d, J=22.0 Hz, 3H, Me), 1.29, 1.23 [ss, 18H, C(Me)₃].

Example 7 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) 4-Methylmorpholine N-oxide as Oxidant with Osmium Catalyst

To a stirred solution of compound 41 (214 mg, 0.1 mmol) in t-BuOH under argon was added a solution of 4-methylmorpholine N-oxide (0.47 mL, 50 wt % solution in H₂O) and water (0.2 mL). A 2.5 wt % solution of osmium tetraoxide in tert-butyl alcohol (0.51 mL) is added, and the mixture is stirred for 5 h at room temperature in a water bath. The mixture is evaporated in vacuo to a syrup, which is azeotroped with H₂O (3×10 mL) to remove 4-methylmorpholine. The residue is dried by addition and evaporation of EtOH (2×10 mL) to give a residue, which was purified by silica gel column chromatography with 20% EtOAc in hexanes to provide the desired product and its isomer (196 mg, 79%) as a solid. Proton NMR indicates that the ratio of the desired product to its isomer is around 5:1. Recrystallization of the mixture from hexanes/ethyl acetate gives pure product (91 mg, 37.4% from starting material) as a crystalline solid. ¹H NMR (DMSO-d₆) δ 1.18 (t, J=7.2 Hz, 3H, —OCH₂ CH ₃), 1.24 (s, 3H, CH₃), 1.25 (s, 3H, CH₃), 1.28 (s, 3H, 2-CH₃), 3.67 (t, J=7.2 Hz, 1H), 3.85, 4.06 and 4.12 (m, 4H), 4.97 (s, 1H, 2-OH, D₂O exchangeable), 5.14 (d, J=7.6 Hz, 2-OH, D₂O exchangeable).

Example 8 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1, 3] dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Ferricyanide as Oxidant with Osmium Catalyst

A 100 mL round-bottomed flask, equipped with a magnetic stirrer, is charged with 5 mL of tert-butyl alcohol, 5 mL of water, and a mixture of K₃Fe(CN)₆ (0.98 g), K₂CO₃ (0.41 g), and K₂OsO₂(OH)₄ (3.2 mg). Stirring at room temperature produced two clear phases; the lower aqueous phase appears bright yellow. Methanesulfonamide (95 mg) is added at this point. The mixture is cooled to 0° C. whereupon some of salts precipitate out, 214 mg (1 mmol) of the compound 41 is added at once, and the heterogeneous slurry is stirred vigorously at 0° C. for 24 h. To the mixture is added solid sodium sulfite (1.5 g) while stirring at 0° C., and then the mixture is allowed to warm to room temperature and stirred for 30-60 min. Ethyl acetate (10 mL) is added, and after separation of the layers, the aqueous phase is further extracted with EtOAc. The organic layer is dried over Na₂SO₄ and concentrated to dryness. The residue is purified by silica gel column chromatography with 20% EtOAc in hexanes to provide the product (190 mg, 77%) as a solid, proton NMR indicates that the ratio of the desired product to its isomer is around 5:1. Recrystallization of the mixture with hexanes/ethyl acetate gave pure diol product (102 mg, 41% from starting material) as a crystalline solid. The ¹H NMR spectrum of this product is identical to that of an authentic specimen.

Example 9 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1, 3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) t-Butylhydroperoxide as Oxidant at Room Temperature with Osmium Catalyst

A 50 mL of flask, equipped with magnetic stirrer, is charged with 2 mL of acetone, 214 mg (1 mmol) of compound 41, 65 mg of Et₄NOAcc.4H₂O, and 0.3 mL of tert-butyl hydroperoxide (5˜6 M in decane). After stirring at room temperature until the Et₄NOAc a clear solution is obtained, the resulting solution is cooled in an ice bath and 5 mL of OSO₄ (2.5 wt % in t-BuOH) is added in one portion. The solution immediately becomes brownish purple. After 1 h the ice bath is removed and the reaction mixture is allowed to warm to room temperature and stirred for 14 h. The rest of the procedure is done exactly the same way as described above. After flash column chromatography, 178 mg (72%) of product is obtained as a solid. In an expanded ¹H NMR, a tiny bump is observed at δ 1.26 indicating the presence of an isomer in less than 4% in the product.

Example 10 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) t-Butyhydroperoxide as Oxidant at 0° C. with Osmium Catalyst

A 250 mL of flask, equipped with magnetic stirrer, is charged with 20 mL of acetone, 2.14 g (10 mmol) of compound 41, 650 mg of Et₄NOAc.₄H₂O, and 3 mL of tert-butyl hydroperoxide (5˜6 M in decane). After stirring at room temperature until the Et₄NOAc has dissolved, the resulting solution is cooled in an ice bath and 5 mL of OsO₄ (2.5 wt % in t-BuOH) is added in one portion. The solution immediately becomes brownish purple. The reaction mixture is then stirred at 0° C. for 6.5 h (monitored by TLC, hexanes:ethyl acetate=4:1, Rf=0.18). Ether (40 mL) is added at 0° C. and the resulting mixture is treated with 5 mL of freshly prepared 10% NaHSO₃ solution in one portion. The ice bath is removed and stirring is continued for 1 h. EtOAc (100 mL) and H₂O (50 mL) are added to the mixture. After separation of the layers, the aqueous phase is further extracted with EtOAc. The organic layer is washed with brine, dried (MgSO4) and concentrated. The residue is purified by a flash silica gel column chromatography with 20% EtOAc in hexanes to provide the product (2.16 g, 87%) as a solid. No contamination of an isomer is detected in this product by vigorous ¹H NMR analyses.

Example 11 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1, 3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Tetradecyltimethylammonium Permanganate (TDTAP) as Oxidant

To a stirred solution of compound 41 (214 mg, 1 mmol), in t-BuOH (10 mL) and CH₂Cl₂ (2 mL) at room temperature is added a solution of KOH (6 mg, 0.1 mmol) in water followed by TDTAP (0.420 g, 1.12 mmol) in small portions over a period of five minutes. TLC after 5 minutes showed that the reaction is complete. The solution is quenched by using 10 mL of saturated sodium bisulfite. The reaction mixture is concentrated in vacuo and the residue extracted with ethyl acetate (3×15 mL), dried (Na₂SO₄), evaporated to give a white solid, which is further dissolved in 5 mL of CH₂Cl₂, passed it through a plug of silica gel topped with Celite, washed with ethyl acetate (50 ml). The filtrate is dried in vacuo to give viscous oil (174 mg 71% yield) as an 8:1 mixture of which the predominant isomer is the titled compound.

Example 12 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Permanganate as Oxidant with 18-Crown-6 Ether—A (at −40° C.)

To a solution of compound 41 (214 mg, 1 mmol) in CH₂Cl₂ (10 mL) and 18-crown-6-ether (37.5 mg, 0.1 mmol) is added KMnO₄ (158 mg, 1 mmol) in portions at −40° C., and the mixture stirred for 2 h at the same temperature. During this time the reaction mixture turns to dark brown. After the reaction was complete, mixture is quenched with saturated solution of sodium bisulfite (10 mL). The resulting colorless mixture is filtered through a frit, and extracted the filtrate with ethyl acetate (2×25 ml), dried (Na₂SO₄) and concentrated to give a viscous oil consisting of 10-20% of unreacted olefin starting material along with the desired diols and its isomer in a ratio of 6:1 (¹H NMR). Olefin starting material can be removed by passing through a small pad of silica gel using 5% ethyl acetate:hexane. A 6:1 mixture of the desired diols is eluted from the column with 20% ethyl acetate/hexane, and obtained as a white solid (200 mg˜80%) upon evaporation of the solvent.

Example 13 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Permanganate as Oxidant with 18-Crown-6 Ether—B (at −10° C.)

To a solution of compound 41 (214 mg, 1 mmol) in CH₂Cl₂ (10 ml) is added 37.5 mg (0.1 mmol) of 18-crown-6-ether, and mixture is cooled to −10° C. KMnO₄ (237 mg, 1.5 mmol) is added in portions, and the mixture stirred at −10° C. for 2 h. During this time the reaction mixture turns to dark brown, which is treated with saturated solution of sodium bisulfite (10 mL). The resulting mixture is filtered through a frit, and the filtrate is extracted with ethyl acetate (2×25 ml), dried (Na₂SO₄) and evaporated to give a white solid (240 mg, 94.4%) consisting of the desired product and its isomer in a ratio of 6:1.

Example 14 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Permanganate as Oxidant in 1:9 H₂O/t-BuOH

To a solution of compound 41 (214 mg, 1 mmol) in t-BuOH (9 mL) and H₂O (1 mL) at 0° C. is added KMnO₄ (237 mg, 1.5 mmol) in portions and the mixture stirred at the same temperature for 2 h. An additional amount (79 mg, 0.5 mmol) of KMnO₄ is charged and the mixture is stirred for another 30 minutes. After work up as above, 128 mg (50%) of a mixture of isomers in a ratio of 8:1 is obtained as a white solid in which the major component is the desired product.

Example 15 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Permanganate as Oxidant in 9:1 H₂O/t-BuOH

To a solution of compound 41 (214 mg, 1 mmol) in H₂O (9 mL) and t-BuOH (1 mL) at 0° C. is added KMnO₄ (237 mg, 1.5 mmol) in portions and stirred at the same temperature for 30 minutes. During this time the mixture turns to dark brown. Saturated solution of sodium bisulfite (10 mL) is added to the mixture, which is filtered, and the filtrate is extracted with ethyl acetate (3×25 ml), dried (Na₂SO₄), and concentrated to give a 4:1 mixture of diol isomers as a white solid (128 mg, 50%), in which the titled compound is the major component.

Example 16 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Permanganate as Oxidant in H₂O at 0° C.

A solution of KMnO₄ (158 mg, 1.0 mmol) in H₂O (10 mL) is added to compound 41 (214 mg, 1 mmol), and the mixture is stirred at 0° C. for 1 hour. The reaction mixture is quenched with saturated solution of sodium bisulfite (10 mL), and the mixture is worked up as above. A white solid (80 mg, 32%) that is obtained is a 4:1 mixture of diol isomers in which the titled compound is the predominant component.

Example 17 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Permanganate as Oxidant in Acetone

To a solution of compound 41 (214 mg, 1 mmol) in acetone (10 mL) is added 37.5 mg, 0.1 mmol) and cooled the reaction mixture to 0° C. To this cold solution is added KMnO₄ (237 mg, 1.5 mmol) in portions, and the reaction mixture is stirred for 2 h at the same temperature. During this time the reaction mixture turns to dark brown. The reaction mixture is quenched with saturated solution of sodium bisulfite (10 ml) where the solution becomes colorless. The reaction mixture is extracted with ethyl acetate (3×25 ml), dried and evaporated the mixture to give a white solid (245 mg, 96.4%) in the ratio of 10:1.

Example 18 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) Potassium Permanganate as Oxidant in a Mixture of Acetone and Pyridine

To a solution of compound 41 (214 mg, 1 mmol) in a mixture of acetone (9 mL) and pyridine (1 mL) at 0° C. is added KMnO₄ (158 mg, 1.0 mmol) and stirred at same temperature for 1 hr. After work up of the reaction mixture as above, 164 mg (67%) of white solid which is practically pure product. Vigorous ¹H NMR analyses reveal the crude white solid contains about 6% of the diastere-omer of the titled compound.

Example 19 (2S, 3R)-3-[(4R)-2,2-Dimethyl-[1,3]dioxolan-4-yl]-2,3-dihydroxy-2-methyl-propionic Acid Ethyl Ester (42) in the RuCl₃/CeCl₃/NaIO₄ System

In a 50 mL round-bottomed flask equipped with magnetic stirring bar, a mixture of NaIO₄ (321 mg, 1.5 mmol) and CeCl₃. 7H₂O (37 mg, 0.1 mmol) in 0.45 mL of water is stirred and gently heated until a bright yellow suspension is formed. After cooling to 0° C., EtOAc (1.25 mL) and acetonitrile (1.5 mL) are added and the suspension is stirred for 2 minutes. A 0.1 M aqueous solution of RuCl₃ (25 μL) is added and the mixture is stirred for 2 minutes. A solution of the compound 41, (214 mg, 1 mmol) in EtOAc (0.25 mL) is added in one portion and the resulting slurry is stirred at 0° C. for 1 hour. Solid Na₂SO₄ (0.5 g) is added followed by EtOAc (3 mL). The solid is filtered off, and the filter cake is washed several times with EtOAc. Then the filtrate is washed with saturated Na₂SO₃ solution and the organic layer is dried (Na₂SO₄) and concentrated to dryness. The residue is purified by silica gel column chromatography with 20% EtOAc in hexanes to provide a syrup (150 mg, 60%). ¹H NMR indicates that the ratio of the desired product to its isomer is approximately 1.6:1.

Example 20 Reduction and Acylation of Compound 49

To a solution of 3,5-dibenzoyl-2-fluoro-2-deoxy-2-methyl-D-ribono-lactone (49, 23 g, 61.77 mmol, scheme 6) in anhydrous THF (400 ml) was added LiAl (^(t)-OBu)₃H (75 mL 1M in THF, 75.0 mmol) over a period of 15 min at −20 to −10 oC and the resulting solution was stirred at the same temperature until all the starting material was consumed. After 5 hours, −10-20% starting material was left, therefore additional 10 mL of LiAl(t-OBu₃H (10 mmol) was added at the same temperature and stirred for an hour when TLC indicated all starting material was consumed. To this reaction mixture were added DMAP (7.5 g) and Ac₂O (58.2 g, 616 mmol) and the solution was stirred at −10° C. for ˜2-3 h. Upon completion of reaction (as indicated by TLC) the reaction mixture was diluted with ethyl acetate (400 ml) and 200 ml of water. The organic layer was separated and the aqueous layer was washed with ethyl acetate (2×100 ml). The combined organic layer was washed with water (3×150 ml), brine and dried over anhy. Na₂SO₄. The solvent was removed under reduced pressure and coevaporated with toluene (2×100 mL) to give crude acetate as a clear brown oil. This oil was passed through a plug of silica gel (50 g) and washed with 20% ethyl acetate/hexanes until all the acetate was recovered. The solvent was evaporated under reduced pressure to give the desired acetate (54, 32 g) as a colorless oil.

Example 21 1-(2-deoxy-2-fluoro-2-methyl-3-5-O-dibenzoyl-fl-D-ribofuranosyl)-N4-benzoylcytosine (55)

To a suspension of N⁴-benzoylcytosine (20.39 g, 94.74 mmol) in 400 ml of HMDS was added (NH₄)2SO₄ (250 mg) and heated under reflux for 4 h. Excess HMDS was removed under reduced pressure. The oily residue was dissolved in chlorobenzene (1 L). To this solution were added a solution of the acetate (25 g) in chlorobenzene (250 mL) and SnCl₄ (190.4 mmol, 49 g) and the mixture was stirred at room temperature for 2 h followed by heating at ˜65° C. for 16 h. The reaction mixture was cooled to 0° C. to which NaHCO₃ (96 g, 1.14 mol) and ethyl acetate (500 ml) were added followed by careful addition of water (20 ml). This mixture was allowed to stir at room temperature for 30 min. The mixture was filtered under vacuum, the residue washed with ethyl acetate. The organic layer was washed with water, brine (2×250 mL) and dried over anhydrous Na₂SO₄. Solvent was removed under reduced pressure to give a pale yellowish-brown solid. This was dissolved in MeOH (250 mL) heated under reflux for 30 minutes, cooled to room temperature and filtered, to give the desired product (55, 8.0 g) as a off-white solid.

Example 22 1-(2-deoxy-2-fluoro-2-C-methyl-fl-D-ribofuranosyl)cytosine (14)

A suspension of 55 from Example 21 (16.7 g, 30.8 mmol, scheme 6) was treated with methanolic ammonia (750 mL, 7M in MeOH) and stirred at room temperature for 12 h and concentrated to dryness under reduced pressure to give pale yellow solid. THF (400 mL) was added to the solid and heated under reflux for 30 minutes and cooled to room temperature. The solid formed was collected by filtration and washed with THF to give 14 (6.7 g, 88%) as an off-white powder. 

The invention claimed is:
 1. A compound of formula 51B:

wherein: each R¹ is independently (C₁-C₆) alkyl, phenyl or benzyl; or two R¹ with the carbon attached thereto form a cyclopentyl or cyclohexanyl group; R² is hydrogen, (C₁-C₆) alkyl, hydroxymethyl, methoxymethyl, halomethyl, vinyl, halovinyl, ethynyl, haloethynyl, allyl or haloallyl; R³ is (C₁-C₆) alkyl, hydroxymethyl, methoxymethyl, halomethyl, vinyl, halovinyl, ethynyl, haloethynyl, allyl or haloallyl; R⁴ is hydrogen, aryl, arylalkyl or (C₁-C₆) alkyl; Nu is F, Cl, Br, N₃, CN, NO₃, CF₃, SCN, OR or NR₂ where each R is independently arylalkyl or (C₁-C₆) alkyl; and M⁺ is tetrabutylammonium, tetraethylammonium, tetramethylammonium, sodium, potassium, cesium, rubidium or silver.
 2. A compound of formula 52B:

wherein: each R¹ is independently (C₁-C₆) alkyl, phenyl or benzyl; or two R¹ with the carbon attached thereto form a cyclopentyl or cyclohexanyl group; R² is hydrogen, (C₁-C₆) alkyl, hydroxymethyl, methoxymethyl, halomethyl, vinyl, halovinyl, ethynyl, haloethnyl, allyl or haloallyl; R³ is (C₁-C₆) alkyl, hydroxymethyl, methoxymethyl, halomethyl, vinyl, halovinyl, ethynyl, haloethynyl, allyl or haloallyl; R⁴ is hydrogen, aryl, arylalkyl or (C₁-C₆) alkyl; and Nu is F, Cl, Br, N₃, CN, NO₃, CF₃, SCN, OR or NR₂ where each R is independently arylalkyl or (C₁-C₆) alkyl. 